A prior art single use syringe has a barrel with a needle, a piston mounted in the barrel for reciprocation, a piston rod mounted in the barrel for moving the piston, and a means for detachably connecting the piston to the piston rod (U.S. Pat. No. 5,389,075). The means for detachably connecting the piston rod to the piston comprises a spring installed between the piston rod and an end wall of the piston and a collar in the form of a half-ring that makes a positive connection between the piston rod and the piston. In this syringe, piston rod is mechanically separated from the piston at the end of it injection stroke, when the spring is compressed to insure a movement of the piston rod relative to the piston. This movement results in a slot between the piston and piston rod opening widely enough for the collar to fall out. As the collar is the only part holding the piston rod and piston together, the piston will remain stationary if an attempt is made to move the piston rod back toward the open end of the barrel so as to fill the syringe for a new injection.
The above-described prior art system has many disadvantages. First, the disconnection of the piston rod from the piston is envisaged during the injection stroke of the piston. This means that some fluid to be injected may be left in the syringe in the event a resistance to injection is greater than the force of spring. The system is not hundred percent proof because fall-out of the collar is not guaranteed as the collar disengages by its own weight. In other words, there is no positive-action disengagement. The collar can get between the piston and the front end of the barrel to hinder the piston movement. In such an event a part of fluid will be left in the barrel and cannot be injected The syringe is not absolutely safe as the piston can be moved back by putting the piston rod askew in the barrel so as to jam the end of the piston rod in the piston for pulling the piston back. The prior art syringe is rather complicated and have too many parts which is not desirable taking into account mass-scale production of such items.